Thermoplastic vulcanizates (“TPVs”) are a class of thermoplastic compositions that include finely dispersed cross-linked elastomer particles forming a disperse phase in a continuous thermoplastic phase. TPVs have the benefit of the elastomeric properties provided by the elastomer phase, with the processability of thermoplastics. TPVs may be produced by a process that includes dynamic vulcanization—a process of selectively crosslinking (otherwise referred to alternatively as curing or vulcanizing) the elastomer component during its melt mixing with the molten thermoplastic under intensive shear and mixing conditions within a blend of at least one non-vulcanizing thermoplastic polymer component while at or above the melting point of that thermoplastic. See, for example U.S. Pat. Nos. 4,130,535; 4,594,390; 6,147,160; 7,622,528; and 7,935,763, the entirety of each of which is incorporated by reference herein.
Conventional plastic processing equipment can extrude, inject, or otherwise mold, and thus press and shape, TPVs into useful products. These thermoplastic vulcanizates can be made light in weight and attractive, with good durability, and can be reprocessed at the end of their product life to produce a new product. For these reasons, thermoplastic vulcanizates are widely used in industry, for example as auto parts, such as dashboards and bumpers, air ducts, weatherseals, fluid seals, and other under the hood applications; as gears and cogs, wheels and drive belts for machines; as cases and insulators for electronic devices; as fabric for carpets, clothes and bedding and as fillers for pillows and mattresses; and as expansion joints for construction. They are also widely used in consumer goods, being readily processed, capable of coloration as with other plastics, and providing elastic properties that can endow substrate materials, or portions thereof, for instance harder plastics or metals, in multi-component laminates, with a “soft touch” or rebound properties like rubber.
Thermoplastic vulcanizates can be prepared by dynamic vulcanization in Banbury mixers, roll mixers and other types of shearing, melt processing mixers. Because of the advantages of a continuous process, such materials can be prepared in single screw or multi-screw extruders.
The environment in which thermoplastic vulcanizates are formed and in which vulcanization of the rubber constituent occurs is typically defined by significant shearing forces, heat, and the presence of a variety of additives, including rubber curing agents and coagents that facilitate cross-linking of the rubber. The processing conditions and selection of materials to be included in the TPV can materially impact the quality of the TPV on extrusion. It is desirable to provide TPVs that exhibit good physical properties and processability, while maintaining excellent extrusion properties. However, balancing these desirable characteristics has proven difficult. Extrusion surface smoothness (which may also refer to as extrusion surface roughness (ESR)) is a particularly important extrusion property as the ESR may dictate the suitability and aesthetics of a final extruded product. At the same time, there continues to be significant room for improvement of TPV processability, such as extrusion throughput rate. Thus, it is desirable when formulating and processing TPVs to maintain or improve ESR in combination with efforts to improve physical properties and processability, such as extrusion throughput rate.
The present invention is directed to compositions and methods for improvement of processability of TPVs, including extrusion throughput rate, while also maintaining or improving physical characteristics such as surface smoothness. In certain embodiments, the invention provides TPVs, and methods for making TPVs, that include an at least partially cross-linked elastomer forming a disperse phase within a continuous phase thermoplastic resin, and that further include a masterbatch additive. The masterbatch additive may include carbon black and/or another additive or additives dispersed in a carrier resin. Carbon black, for example, is typically used to impart a black color to a TPV, and may also impart UV-resistant properties to the TPV. Although carbon black and/or other additive(s) may be dispersed within a polypropylene carrier resin to form the masterbatch to be added to the TPV, the present inventors have surprisingly found that propylene- or ethylene-based copolymers are superior carrier resins for masterbatches incorporated into TPVs. In particular, when certain propylene- or ethylene-based copolymers are used as carrier resins in a masterbatch added to TPV formulations, the resulting TPVs exhibit enhanced surface smoothness and faster extrusion throughput, among other beneficial properties. Even more surprisingly, these effects are enhanced even as compared to TPVs that include propylene- or ethylene-based copolymers blended directly into the TPV formulation.